Q
We have started a new project and decided to use Analog Devices digital
isolators instead of opto-couplers. But we have some doubts. I have a few
technical questions concerning ADUM1400 digital isolator.
1- In data sheet, a maximum of 1ms input rise-fall time is recommended. What
happens if I have an input with a rise or fall time more than 1ms? In our
application we may have such rise-fall times. Is Oscillation or an unknown
state possible? Could you please give more information about input stage of the
IC?
2- Does ADUM1400 have output short circuit protection feature? If no, what
precautions can be taken for output short circuit protection?
3- Could you please give me information about max. input clamped current?
A
1- In data sheet, a maximum of 1ms input rise-fall time is recommended. What
happens if I have an input with a rise or fall time more than 1ms? In our
application we may have such rise-fall times. Is Oscillation or an unknown
state possible? Could you please give more information about input stage of the
IC?
Answer: The internal circuit for the inputs has about 200mV of hysterisis.
When the input is going "through the threshold zone" the best case is to get
the signal through this zone as fast as possible, if the signal lingers in this
zone then there is a chance the part will oscillate. If the input is clean (
less than 200mV of noise) then hysterisis will protect against oscillation. So
you will need to know how much noise is on your input to see if you can
tolerate a slower signal rate of change.
2- Does ADUM1400 have output short circuit protection feature? If no, what
precautions can be taken for output short circuit protection?
Answer: Short circuit is safe guarded against by adding a resistor in series
with the output. Another way would be to add a resistor in series with Vdd.
We do not recommend exposing an output pin to a short circuit without the above
protection.
The higher the value the better.
3- Could you please give me information about max. input clamped current?
Answer: The main spec to look at here is really max input voltage. The input
current will increase significantly if Vin > 0.5V+Vdd, at this point the input
clamp diodes start to turn on. The input circuit consists of ~250 Ohm in
series with clamp diodes to the rail and ground. We recommend if the voltage
at the input is going to exceed the max rating, that you put a resistor in
series with the input pin to adhere with the max voltage rating spec. You can
calculate the value based on your applications max Vin.